Powder spreaders play an important role in the initiation and creation of powder beds that are used in layered manufacturing processes known as free form fabrication processes. In free form fabrication processes, the powder bed is formed layer by layer upon a support surface. Typically, the thickness of the powder layer is about the same as the average powder particle thickness of the powder which is being spread or a multiple thereof. Confining walls are supplied beforehand or constructed in situ as the process proceeds to retain the powder layers in place to form the bed. After a powder layer is spread, powder particles in selected portions of the powder layer may be bonded together and/or to an underlying layer through the selective exposure of the powder bed to radiation and/or a fluid to form a two-dimensional slice of one or more three-dimensional objects. This step of selective exposure of a powder layer is sometimes referred to in the art as “printing” and the layer after printing is referred to as a “printed layer,” regardless of the agent to which the powder layer is being selectively exposed. The free form fabrication layer proceeds layer by layer until the entire three-dimensional object or objects have been printed.
Some examples of free form manufacturing processes are the three-dimensional printing (“3DP”) process and the Selective Laser Sintering (“SLS”) process. An example of the 3DP process may be found in U.S. Pat. No. 6,036,777 to Sachs, issued Mar. 14, 2000. An example of the SLS process may be found in U.S. Pat. No. 5,076,869 to Bourell et al., issued Dec. 31, 1991.
It is critical to the success of free form fabrication processes that each powder layer approximate a flat sheet of uniform thickness and density so that each printed layer corresponds geometrically to the intended two-dimensional slice of the object that is being created. Inasmuch as each layer is often on the order of just a few thousands of an inch thick (roughly, a few scores of microns), it is not uncommon for an object to be made from hundreds, or even thousands, of such layers. Even small distortions in the thickness or density uniformity of each layer can add up to substantial distortions in the free form fabricated object.
Until now, nearly all free form fabrication processes have used one of three types of powder spreaders. One type relies on the powder dispenser to dispense a uniform layer of power as it travels across the bed. Examples of this type are found in U.S. Pat. No. 7,828,022 B2 to Davidson et al., U.S. Pat. No. 6,672,343 B1 to Perret et al. and U.S. Patent Publication No. US 2010/0272519 A1 of Ederer et al. A second type comprises a blade which is at least as long as the intended powder bed is wide that is wiped across the powder bed surface to spread out powder that is deposited by a powder dispenser. Examples of this type of powder spreader are found in U.S. Pat. No. 5,387,380 to Cima et al. and U.S. Pat. No. 6,799,959 B1 to Tochimoto et al. The third type comprises a roller which is at least as long as the intended powder bed is wide that is traversed across the powder bed surface to spread out powder that is deposited by a powder dispenser. The roller is rotated in a direction which is opposite to that which the roller would rotate if it were simply being rolled across the powder bed surface. Such “counter-rotation” roller powder spreaders have been found to give superior results to the blade powder spreaders because the rotating action of the roller picks up and redistributes the dispensed powder in front of the roller as it is encountered instead of just pushing the powder pile thus better overcoming the distribution disparities of the as-deposited powder. The rotation action at the trailing side of the roller provides a consistent gentle compaction of the powder. Examples of the counter-rotating spreaders are given in U.S. Pat. No. 5,597,589 to Deckard and U.S. Patent Publication US 2001/0050448 A1 of Kubo et al.
Counter-rotation rollers have their limitations. The rollers are supported and driven at their ends. Typically, they are made of hardened steel or coated aluminum and are precision ground to provide concentricity and straightness. They are also provided with a surface finish that is conducive to the front of the roller lifting and the trailing portion of the roller compacting the powder with which it is to be used. Their diameters are kept small, e.g., on the order of less than 2 inches (5.1 cm), because the compaction force of the trailing side of the roller increases as the roller diameter increases and too much compaction force may degrade or destroy the printed powder bonds of the underlying printed layers. Although short counter-rotation rollers have proven to be effective, as they become longer to accommodate larger powder beds, their small diameters tend to result in increasing amounts of wobble and flexing of the roller during use which compromises the flatness and uniformity of the powder bed. For example, a twelve inch long, two-inch diameter roller was measured to have 0.002 inches of wobble (50 microns), which can be an intolerable amount for layer thicknesses on the order of 0.003 inches (76 microns).
Individual counter-rotation rollers have the further disadvantage of lacking versatility with regard to the types and sizes of powders with which they can be used. As mentioned above, their diameters and surface finishes are tailored to provide the desired balance of powder lift and compaction for the particular types of powders with which they are expected to be used. Also, their electrical conductivities and magnetic properties are fixed and this further restricts the types and sizes of powders with which they can be optimally used inasmuch as even small electrostatic and magnetic forces can have large attractive or repulsive effects on individual powder particles. Moreover, the roller's leading surface, i.e., the portion of the roller's surface that is in contact with the powder that is ahead of the direction of the roller's travel across the bed (the “leading powder”), has the same contour as the roller's trailing surface, i.e, the portion of the roller's surface that is in contact with the powder that is in the direction opposite to the roller's direction of travel across the bed (the “trailing powder”). This configuration precludes the independent control of the lift and compaction provided by the roller.